This invention relates to vehicles having hydraulic fluid and compressed air sources and to a braking system for the vehicle, and more particularly to a braking system wherein a single braking device is used for service braking, for retarding, for parking and for emergency operation.
Brake cylinders having a spring-loaded piston therein have been used for such purpose, the piston being biased to a brake-applying position by the spring so that if there is insufficient pressure available from the vehicle pressure system, the brakes will be automatically applied. During normal operation, hydraulic fluid from the fluid system of the vehicle is applied to the brake cylinders to move the piston therein in a direction so that the vehicle brakes are released. The operator can then apply the brakes in service, retarding or parking modes by regulating the resultant fluid pressure in the cylinder on the spring-biased piston therein so that the piston will move either towards brake-releasing or brake-applying positions.
In order to carry out their emergency function, such brake systems are generally designed so that a loss of actuating pressure will cause an automatic application of the brakes. However, rather than having an abrupt braking in the event of emergency, the operator will normally prefer to bring the vehicle to a smooth and controlled stop by means of use of the usual foot pedal normally used for service braking.
Once the disabled vehicle has been brought to a halt it may be necessary to tow the vehicle to a suitable destination for repair work. It is desirable that the brakes can be released without using the normal primary pressure so that the vehicle can be towed. It is also desirable that service and parking brake modes of operation can be carried out while the vehicle is disabled. In order that the operator not have to learn a separate procedure for emergency situations it is preferable that the service and parking brake operations be carried out with the same operator controls and in the same manner whether the vehicle is operating normally or in emergency mode.